Dislocation evolution during tensile deformation in ferritic-martensitic steels revealed by high-energy X-rays

Deformation processes in Grade 91 (Fe-9%Cr-1%Mo-V,Nb) and Grade 92 (Fe-9%Cr-0.5%Mo-2%W-V,Nb) ferritic-martensitic steels were investigated at temperatures between 20 and 650 °C using high-energy synchrotron X-ray diffraction with in situ thermal-mechanical loading. The change of the dislocation density with strain was quantified by X-ray diffraction line profile analysis complemented by transmission electron microscopy measurements. The relationship between dislocation density and strain during uniform deformation was described by a dislocation model, and two critical materials parameters, namely dislocation mean free path and dynamic recovery coefficient, were determined as a function of temperature. Effects of alloy chemistry, thermal-mechanical treatment and temperature on the tensile deformation process in Grade 91 and Grade 92 steels can be well understood by the dislocation evolution behavior.